Release 10 of the 3GPP standard specifications—the Evolved UMTS Terrestrial Radio Access Network or E-UTRAN standard—introduced Carrier Aggregation or CA as a means for qualifying E-UTRAN to meet the requirements for 4G services of up to 1000 Mbit/s, as well as for allowing operators with small, scattered spectrum allocations, e.g., allocations of 20 MHz or less, to provide a good user experience, based on aggregating the scattered allocations into aggregated allocations of 10, 20 MHz or more.
In the context of CA operation, a user equipment or UE is connected to a serving cell that is termed the Primary Cell, PCell, on what is referred to as the Primary Component Carrier or PCC. Mobility is managed with respect to the PCC, but in cases where the UE is using services that require high throughput, the network may activate one or more additional serving cells. Each additional serving cell is termed a Secondary Cell, or SCell, on what is referred to as a Secondary Component Carrier or SCC. The activation may happen before or after the UE detects the SCell.
Release 10 of the 3GPP standards consider and define two types of aggregation scenarios: intra-band contiguous aggregation and inter-band aggregation. Release 11 of the 3GPP standards further consider intra-band non-contiguous aggregation, while Release 12 of the 3GPP standards further considers aggregation of three downlink, DL, carriers, with one or two uplink, UL, carriers. These carriers may be inter-band or intra-band, contiguous or non-contiguous, or any combination thereof. Release 12 further considers the aggregation of Frequency Division Duplex, FDD, carriers with Time Division Duplex, TDD, carriers, where the PCC and any one or more SCCs to FDD and TDD, or to TDD and FDD, respectively.
For intra-band contiguous carrier aggregation the PCell and SCell(s) are contiguous in frequency. The applicable 3GPP standards requires that for contiguous intra-band aggregation, the time difference between the PCell and an SCell is allowed to be at most ±130 ns—see 3GPP TS 36.104 rev 11.4.0, sub-clause 6.5.3. The standard further assumes that for this particular scenario, the involved receiver can use a single fast Fourier transform, FFT, circuit or operation to demodulate the signal from both the PCell and the SCell simultaneously. Thus in practice it is required that the PCell and SCell are co-located, i.e., transmitted from the same physical network node site, otherwise propagation delay would make it impossible to use a single FFT circuit or operation.
For intra-band non-contiguous aggregation, the timing difference is allowed to be at most ±260 ns, but cell co-location is not assumed, nor is it assumed that a single FFT can be used. Similarly, for inter-band carrier aggregation the timing difference between the PCell and a SCell is allowed to be at most ±260 ns. However, the inter-band scenario further assumes that the cells may be non-co-located and that the UE will have to cope with a propagation delay difference between the PCell and the SCell of up to ±30 μs, resulting in a maximum delay spread of ±30.26 μs—see 3GPP TS 36.300, revision 11.5.0, Annex J.
FIG. 1 illustrates example carrier aggregation deployment scenarios (a) through (e). In particular: item (a) illustrates co-located overlaid intra-band scenario where there is similar path loss for different carriers; item (b) illustrates co-located overlaid inter-band scenario where there is different path loss for different carriers; item (c) illustrates co-located inter-band partially overlaid scenario, item; (d) illustrates non-co-located remote radio heads (RRH) with inter-band carriers used to provide improved throughput at hotspots; and item (e) illustrates an overlaid inter-band scenario with repeaters. See 3GPP TS 36.300 rev 11.5.0 Annex J.
Thus, FIG. 1 can be understood as illustrating examples of foreseen deployment scenarios that are applicable up to 3GPP Release 11. For the co-located intra-band scenario with fully overlapping coverage of the PCell and SCell, the eNodeB or eNB (LTE base station) can configure and activate the SCell when needed, based on reported measurements for the PCell.
The timing of the SCell is a known value in case the UE has measured and reported the cell recently, either as inter-frequency neighbor cell or as a cell on a configured secondary component carrier F2. Additionally, regardless of having been reported before, the timing of the SCell is also considered as being known in case of intra-band contiguous carrier aggregation, i.e., where the spectrums for the PCell and SCell are back-to-back. When the UE gets an activation command for the SCell under such conditions, the UE may be able to start reception from the SCell without prior fine-tuning of the timing.
In case the cell has not been reported previously and is on another band, i.e., an inter-band scenario, or is non-adjacent, the timing of the SCell is not known to the UE. However, the SCell timing shall fall within ±30.26 μs relative to the PCell. This timing window is significant as it occupies almost half an OFDM symbol time and, in such cases, the timing of the SCell will have to be tuned before the UE can start reception from the SCell.
FIG. 2 illustrates a future deployment scenario. Because of the use of partially overlaid cells in some locations, a UE may have to aggregate one carrier, e.g., F1, from a network node or base station eNB A, and another, e.g., F2, from another network node or base station eNB B. Each network node manages several cells on two carriers. In the diagram, cells on F1 and F2 managed by eNB A and eNB B are labeled A and B, respectively.
From 3GPP Release 12 and onwards, such so-called inter-node radio resource aggregation is under discussion—see e.g. 3GPP TR 36.842. For one of the foreseen scenarios, the UE may be connected to a primary cell, a “master” cell, handled by one base station, and simultaneously to between one and four secondary cells, “assisting” cells, handled by other base station(s). In case the primary cell and secondary cell(s) are on different carriers, the UE handles aggregation in a manner similar to aggregation in the Release 11 deployment scenarios depicted in FIG. 1. One difference, however, is that in scenarios up to 3GPP Release 11, the aggregated cells were handled by the same network node—e.g., the same eNB or other base station—with either co-located cells on different carriers but sent from the same site, or non-co-located cells on different carriers, using Remote Radio Heads, RRHs. Such deployment scenarios are shown in the example items (e) and (f) in FIG. 1.
Thus, FIG. 2 can be understood as depicting one example of inter-node radio resource aggregation/inter-node carrier aggregation. A UE that is in the coverage of network node eNB A on one carrier and in the coverage of network node eNB B on another carrier may aggregate both carriers even though the cells are handled by different base stations. In contrast, aggregation as considered in the 3GPP standards up to Release 11 would only be done within each respective base station, either eNB A or eNB B, but not both eNB A and eNB B. Note that the cells on both carriers may provide macro coverage—i.e., have large cell radius.
3GPP TS 36.133 specifies the requirements on the maximum delay for SCell activation, from reception of the activation command until valid channel state information, CSI, is transmitted to the network. With favorable radio conditions and SINR>−3 dB, activation shall be completed within: 24 ms if the cell is known, which is defined as Reference Signal Received Power (RSRP) measurements having been reported to the network within the last min of 5 DRX cycles or 5 SCell measurement cycles; and 34 ms if the cell is unknown—i.e., a blind activation where the cell has not been reported within the last min 5 DRX cycles or 5 SCell measurement cycles. Here, “DRX” denotes discontinuous reception.
The UE shall start transmitting CSI 8 ms after having received the SCell activation command. Before synchronization to the SCell has been achieved, CSI shall indicate out-of-range, which is indicated using CQI index 0. The requirements shall be met for a worst-case scenario regarding the available number of unicast subframes. For LTE FDD, the worst case is when there are two unicast subframes per 5 ms, For LTE TDD, the worst case is when there is only one unicast subframe and one special subframe per 5 ms.
In dual connectivity, DC, operating scenarios, the UE can be served by two nodes, which are referred to as a “main” eNB or MeNB, and “secondary” eNB or SeNB. The UE is configured with a PCC from both MeNB and SeNB. The PCells from the MeNB and SeNB are referred to as the PCell and the PSCell, respectively. The PCell and PSCell typically operate independently with respect to the UE. The UE is also configured with one or more SCCs from each of the MeNB and SeNB. The corresponding secondary serving cells served by MeNB and SeNB are simply referred to as SCells. A UE operating in DC typically has separate transceivers, TX/RX, for each of the connections with the MeNB and SeNB. This feature allows the MeNB and SeNB to independently configure the UE with respect to one or more procedures on the PCell and the PSCell. Examples of such procedures include radio link monitoring, RLM, DRX cycles, etc.
The UE can be configured to periodically report CQI to the base station. For LTE FDD, the reporting period can be: 2, 5, 10, 20, 40, 80, 160, 32, 64, and 128 ms, respectively. For LTE TDD, the reporting period can be: 1, 5, 10, 20, 40, 80, and 160 ms, respectively. Further, as seen in 3GPP TS 36.213, clause 7.2.2, there are some restrictions on the UL/DL configuration in use. A typical network configuration uses a CQI reporting period in the range 5 to 40 ms.
The CQI values that can be reported are depicted in Table 1, as presented in FIG. 3. More particularly, the table depicts 4-bit CQI values according to 3GPP TS 36.213, clause 7.2.3. Note that according to its conventional meaning, a CQI index value of 0 indicates to the eNB that the UE is out of radio coverage. CQI reporting may be aperiodic, in which case the UE reports CQI to the eNB responsive to indications in the Downlink Control Information or DCI.
Event-triggered reporting also may be used. For example, for mobility measurement purposes, a UE may be configured with events. The triggering of a given event causes the UE to take some action. For example, a certain event trigger causes the UE to report measured signal strength and signal interference values for detected cells. Existing events in E-UTRA are seen in 3GPP TS 36.331, V12.1.0 and include: event A1 in which the serving cell becomes better than threshold; event A2 in which the serving cell becomes worse than threshold; event A3, in which a neighbor cell becomes better than the PCell by some defined offset; event A4, in which a neighbor cell becomes better than some threshold; event A5, in which the PCell becomes worse than a threshold1 and a neighbor cell becomes better than a threshold2; event A6, in which a neighbor cell becomes better than an SCell by some defined offset; event B1, in which an inter RAT neighbor cell becomes better than some threshold; and event B2, in which a PCell becomes worse than a threshold1 and an inter RAT neighbor cell becomes better than a threshold2.
It is recognized herein that existing protocols and techniques do not provide a UE with any means for the UE to indicate to an eNB that the time difference between the PCell and any of the SCells is becoming larger than the UE can handle. The full responsibility is put on the eNB to maintain accurate information on what timing difference the UE may experience. However, the tools for acquiring such information, such as Observed Time Difference Of Arrival, OTDOA, or Reference Signal Time Difference, RSTD, are not available to the eNB, as such techniques are handled by nodes deeper into the core network. While proprietary solutions might be used by the eNB, such approaches would still only involve a predicted time difference. Hence, to ensure that the UE is within an area where carriers can be aggregated, the eNB likely will have to be more conservative than necessary. At the same time there may be UEs that are capable of handling PCell-to-SCell time differences beyond the ±30.26 μs range.
Additionally, in at least some mobility scenarios, particularly in urban areas or hilly terrain, the radio propagation delay may change rapidly. For example, the radio propagation delay changes rapidly and by a potentially significant amount whenever the line-of-sight is lost and the UE receives only reflected radio waves. A UE that has been activated while within the range of supported time differences between the PCell and an SCell may experience a time difference outside that range, particularly if the SCell has been activated close to the border with respect to propagation delays between PCell and SCell. Currently, the behavior of a UE that suddenly falls outside of its supported range of time differences is undefined.
Moreover, it is recognized herein that in inter-node radio resource aggregation new deployment scenarios will be encountered where it is likely that not all timings for cells under which the UE has coverage simultaneously are such that they fall within the time difference that the UE can handle, e.g. ±30.26 μs. Hence some cells will not be suitable to use for aggregation, but the UE has no means for indicating to the network as to which cells can be used for aggregation.